1. Field of the Invention
The present invention relates to an alkaline storage battery using nickel oxide for the positive electrode, and can be applied to nickel-metal hydride storage batteries using a hydrogen-absorbing alloy for the negative electrode and nickel-cadmium storage batteries using cadmium for the negative electrode. The invention particularly relates to a technique for obtaining batteries with enhanced capacity.
2. Description of the Related Art
In recent years, with the widespread use of portable appliances, the demand for small-sized secondary batteries is increasing. Among them, alkaline storage batteries which use nickel oxides for the positive electrode and an aqueous alkaline solution as the electrolyte have been much in demand because of their advantages of low cost, high energy density and stoutness.
Of these batteries, the nickel-metal hydride storage battery has acquired a still more enhanced capacity as compared with the nickel-cadmium storage battery by using for the negative electrode hydrogen absorbing alloys capable of absorbing and releasing hydrogen electrochemically. The alkaline storage batteries, that is, nickel-metal hydride storage batteries and nickel-cadmium storage batteries, are much expected as a battery which can meet a wide range of uses extending from the use for small-sized portable appliances to that for large-sized electric automobiles. In particular, increasing the energy density of batteries, which makes it possible to attain batteries with smaller size and lighter weight, is eagerly required in the market. Prior main techniques used in regard to increasing the energy density of batteries in the above-mentioned battery system are described below.
Nickel hydroxide used as the positive electrode active material of alkaline storage batteries inherently has a very low electric conductivity, but it is converted to a trivalent nickel oxyhydroxide having a somewhat higher conductivity by charging. At the last stage of discharge, however, the content of divalent nickel hydroxide in the active material particles increases and the conductivity of the active material decreases; resultantly, the overvoltage increases and the discharge voltage decreases sharply.
It is already known that, to suppress the above-mentioned phenomenon, an additive comprising mainly cobalt oxide is added to the positive electrode to form on the surface of the nickel hydroxide active material trivalent cobalt oxyhydroxide having a higher effect in increasing the conductivity and thereby to give a high conductivity to the active material as a whole and suppress the above-mentioned increase of overvoltage. This method has made it possible to raise the utilization rate of the active material to 100%.
Further, to improve the utilization rate of the active material still more, as disclosed for example in JP-A-8-148145 and JP-A-8-148146, a method of adding a cobalt compound which has a higher conductivity than previous additives has been proposed.
On the other hand, the improvement of the active material itself to attain a higher capacity has also advanced; as disclosed in JP-A-8-236110, attempts have been made to improve the utilization rate by incorporating manganese, chromium, aluminum, etc. as solid solution into the active material particles comprising mainly nickel oxide. Whereas ordinary nickel hydroxide is changed into .beta.-type nickel oxyhydroxide by charging, the above-mentioned method intends, by the incorporation of manganese, etc. into the active material as a solid solution, to form .gamma.-type nickel oxyhydroxide positively, the formation of which has been apprehended in the prior techniques.
Thus, in contrast to .beta.-type nickel oxyhydroxide, .gamma.-type nickel oxyhydroxide has a large specific volume, hence causes the swelling of the positive electrode plate, and moreover exhibits a fairly low discharge voltage. Therefore, it has been considered important to suppress the formation of .gamma.-type nickel oxyhydroxide to as low a level as possible. Consequently, attempts have been made, for the purpose of suppressing the formation of .gamma.-type nickel oxyhydroxide, to add zinc oxide or the like as an additive to the positive electrode or as a material to be incorporated into the active material to form a solid solution.
However, a recent technique which incorporates manganese into active material particles to form a solid solution has made it possible to raise the discharge voltage of .gamma.-type nickel oxyhydroxide, which has been considered to discharge with difficulty, to the same level as the discharge voltage of .beta.-type nickel oxyhydroxide. Accordingly, there has recently been a trend to positively utilize the .gamma.-type nickel oxyhydroxide for attaining a higher capacity of batteries.
Though it is generally said that the nickel in the .gamma.-type nickel oxyhydroxide in the charged stage assumes an oxidation number of trivalences or more and less than tetravalences, it can be considered that the oxidation number varies somewhat depending on the manners in which alkaline cations and water molecules are incorporated into the interlayer spaces of the nickel oxyhydroxide crystals and in general the oxidation number seems to be a value of about 3.5-valences.
On the other hand, the nickel in the .beta.-type nickel hydroxide in the discharged state is in the divalent state, so that it is considered that in the charge-discharge reaction between the .gamma.-type nickel oxyhydroxide and the .beta.-type nickel hydroxide, 1.5 electrons can move at the maximum per nickel atom. That is to say, whereas the nickel in the prior .beta.-type nickel oxyhydroxide is trivalent and hence one electron moves at the maximum per nickel atom, the .gamma.-type nickel oxyhydroxide has the potential capability to raise the utilization rate of nickel oxide to about 150%.
Separately, it has already been proposed in U.S. Pat. No. 5,523,182 to use an active material comprising a nickel hydroxide active material and at least three compositional modifiers selected from the group consisting of Al, Bi, Co, Cr, Cu, Fe, In, La, Mn, Ru, Sb, Ti and Zn and at least one chemical modifier selected from the group consisting of Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr and Zn thereby to form a cobalt compound on the surface of the active material particles and to form a cobalt capsule layer on the surface at the time of the first charging.
As to the negative electrode, AB.sub.5 type hydrogen absorbing alloys comprising mainly rare earth metals-nickel have heretofore been in wide use but, in place thereof, hydrogen absorbing alloys composed mainly of AB.sub.2 type C14 or C15 Laves phase comprising zirconium and nickel as the main components are attracting attention because of their advantage of high capacity.
However, these techniques for increasing the capacity hitherto proposed are still unsatisfactory for attaining a high capacity in the above-mentioned battery system.
For example, in the method of adding a cobalt compound disclosed in JP-A-8-148145 and JP-A-8-148146, the utilization rate of the active material reaches its upper limit at about 110% and no more improvement in the utilization rate can be expected. This leads also to an unsatisfactory result in attaining a high capacity of the battery as a whole.
In the method of incorporating manganese and zinc into the active material as a solid solution disclosed in JP-A-8-236110, wherein an element which suppresses the formation of .gamma.-type nickel oxyhydroxide, such as zinc, is contained as a solid solution in the active material, the formation of .gamma.-type nickel oxyhydroxide is suppressed and the utilization rate attainable is about 110% at the most also in this case. Thus, the material obtained by this method is still unsatisfactory for attaining a high capacity.
Separately, when the technique disclosed in U.S. Pat. No. 5,523,182 is used, owing to the expansion of the active material caused by the formation of .gamma.-type nickel oxyhydroxide, the network of cobalt oxyhydroxide is broken and the efficiency of electronic conduction between active material particles or between the active material and the core material is lowered, and hence a sufficient utilization rate is difficult to obtain. Moreover, owing to the above-mentioned influence on the conductivity between the active material particles and between the active material and the core material caused by the breakage of the cobalt oxyhydroxide network as well as owing to the lowering of conductivity of the active material itself, the overvoltage rises sharply at the last stage of discharge, and nickel is reduced only to a state of 2.1-valences or higher. Thus, the active material cannot fully exhibit its potential capacity.
Furthermore, replacing the negative electrode material by a high-capacity hydrogen absorbing alloy composed mainly of AB.sub.2 type C14 or C15 Laves phase is also still unsatisfactory for attaining a higher capacity of batteries.
In these nickel-metal hydride storage batteries which have the highest energy density among those actually available on the market at present, the occupied volumes of the constituent members relative to the total volume of the battery are: about 50% for the positive electrode, about 25% for the negative electrode and the remaining 25% for the separator, electrolyte and vacant space; thus the positive electrode occupies a larger volume than the negative electrode. What determines the battery capacity in the battery is the capacity of the positive electrode, and it is indispensable for increasing the battery capacity to increase the amount of the active material of the positive electrode or to improve the utilization rate of the positive electrode. Even if the capacity of the negative electrode, which is relatively small in volume, is markedly improved, the contribution of the improvement to attaining a high battery capacity is actually disappointedly small.
Therefore, for attaining a high capacity of these batteries, increasing the capacity of the positive electrode is the major prerequisite. If the capacity of the positive electrode can be further increased, the volume occupied by the positive electrode in the battery decreases, the occupied volume of the negative electrode can be increased and resultantly the increase of the capacity of the negative electrode becomes of more value. In the prior methods as disclosed in U.S. Pat. No. 4,946,646, their effect is confined to increasing the capacity of the negative electrode alone, and the above-mentioned effect of increasing the battery capacity as a whole by the increased capacity of the negative electrode is not satisfactorily exhibited.
The object of the present invention is, overcoming the above-mentioned difficulties, to provide an alkaline storage battery of a high capacity by selecting the optimum combination of (1) improving the utilization rate of the positive electrode active material itself as well as the electric conductivity between the positive electrode active material particles and between the active material and the core material, to suppress the rise of overvoltage at the last stage of discharge and to make it possible to take out more electricity, and (2) using a negative electrode material of a higher capacity to decrease the occupied volume of the negative electrode.